Contemporary automated machine tools distinguish themselves with relatively high productivity. This productivity results both from the elimination of manual labor and application of high cutting velocity using advanced cutting tool materials. Such factors are especially important in light of the current trend toward a totally unmanned factory in the not-so-distant future.
However, while machining holes in the vast majority of materials used in machine parts today, an unwelcome obstacle is encountered. As a by-product of the hole-making process, continuous chips are formed. These chips create serious problems connected with chip formation during machining. The problems become especially acute in drilling deep holes and high speed hole boring. For instance, in drilling, long bands of the continuous chips press against the machined surface of the hole as they move along the drill flutes, increasing the roughness of the machined surface. The chips also tend to clog tool flutes. When this occurs, feed motion must be stopped so that the bands of tangling chips can be removed. Still further, these chips have a tendency to accumulate in the limited working space surrounding a tool. This is difficult to overcome and is an obstruction to the efficient utilization of automated tooling. Moreover, the chips frequently cause, if left unattended, the premature wear and/or failure of the tooling as well as damage to the machined surface. Additionally, continuous chips can become entangled about the tool, the workpiece, and/or rotating elements of the machine tool system.
Generally, unless monitored and removed constantly from the working space, such chips are hazardous to the tool, the machine, the quality of the product, and the operator (if any). Unfortunately, in practice, the removal of whirling chips can usually be accomplished only manually. The feed motion is interrupted from time to time and the tool is withdrawn from the material being drilled. As a result, a long band of chip is broken. But such manual labor applied to automatic machine tools, including NC and CNC machine tools, contradicts fundamental principles of their economic utilization in a manufacturing process. The automated versions of this procedure, such as periodic stopping of the feed motion and/or total or partial withdrawal of the tool from the hole being machined, do not solve the problem since they lengthen the overall drilling operation. Moreover, driving a heavy machine tool headstock or carriage at a periodically varying velocity causes undesirably strong dynamic forces due to the huge masses subjected to periodic accelerations and decelerations, adversely affecting the machine tool, the accuracy of the machining, and, often, the machined surface roughness. These limitations are particularly serious and critical when machining at high rotary tool speeds since the high frequency of the headstock velocity variations causes excessive inertia forces. Other conventional approaches, based on avoiding certain ranges of cutting parameters, inevitably result in partial utilization of expensive automatic machine tools because problems with continuous chips are characteristic of high cutting speed and feed rate. In addition, even the chips produced at lower cutting speeds and feed rates frequently maintain their continuous form. However, by using the apparatus and method of the present invention in machine shop practice, the necessity of manual chip removal is simply avoided.
Conventional chip breakers would have solved such problems long ago if they were effective and reliable. Although they are relatively inexpensive and simple, conventional chip breakers are ineffective beyond the narrow ranges of cutting parameters for which they are designed. Any change in cutting speed, feed rate, and depth of cut affects the performance of conventional chip breakers. In addition, for many materials, usually referred to as "difficult-to-machine" or "space-age materials," they are entirely ineffective. Economically, the most disadvantageous feature of traditional chip breakers is that their use shortens the effective life of a cutting tool, simply because the chip-breaking action is far outlasted by the tool itself.
In the prior art, various modes of vibration applied to the cutting tool have been attempted, especially when drilling, as a means for chip breaking. However, tool vibration alone does not ensure the formation of intermittent chips, since chip breakage is a function of the rotating tool's angular position and position along its feed advance path. Uncontrolled periodic variation of chip thickness may produce chips having a thickness which is only randomly reduced to zero. But this causes equally uncontrolled and random chip separation or breaking.
In order to provide additional background information so that this invention may be completely understood and appreciated in its proper context, reference may be made to the following publication and patent, the disclosures of which are incorporated by reference: C. H. Kahng, et al., "Study of Chip Breaking During Twist Drilling," SME Tech. Papers, Ser. MR at Ann. Conf. - Book 2, Cleveland, Ohio, Apr. 26-29, 1976; U.S. Pat. No. 3,431,799, issued to J. R. Bashor.
C. H. Kahng, et al. presented their study of the fundamental characteristics of chip formation and chip breaking in the twist drilling operation. Results of their investigation confirm the description of the prior art given above. The authors concluded that the size of the chip can be reduced and well-broken chips obtained, when an oscillation is applied between drill and workpiece. They also considered the use of a stepped feed drive for chip breaking, suggesting that the length of the chip could be controlled in this manner. However, the Kahng, et al. paper does not disclose any further details regarding these methods nor does it disclose apparatus capable of carrying out the methods.
U.S. Pat. No. 3,431,799 discloses a chip breaker for the drilling operation. The apparatus is a multiple-spindle drill head, the use of which is limited to a drilling operation. The drill head housing is actuated, so as to produce alternating lowering and elevating movements of the housing and the drills, spindles, and associated driving gears carried thereby. The actuation of the drill head is independent of the feed rate, such that there is no relation between the length or number of chip segments and the tool revolution, as well as between the tool angular position and its position along the feed advance path. It follows that it is impossible to find and use key operating parameters (described in greater detail herein below), such as the optimum phase .phi. and the optimum double amplitude-to-feed ratio 2A/p. The reciprocal movement of the head periodically withdraws cutting edges of all the drills from the material being drilled and interrupts the cutting action of the drills. There are alternate drilling and non-drilling periods. During the non-drilling periods, no chips are produced, resulting in the breakage of continuous chips. The method embodied in this apparatus is essentially an automated version of what is conventionally done manually to break long bands of continuous chips, with all of the attendant disadvantages of this approach previously described.
Upon review of the prior art based on application of tool vibration or oscillation, it may be observed that intermittent chip forming has been obtained only randomly, depending on constantly changing phase .phi. due to absence of the dependence of the tool's axial position on its angular position.
Whatever the precise merits, features and advantages of the known prior art, including the above discussed references, none achieves or fulfills the purposes of the method and apparatus of the present invention. To advance the art, it is preferable that the cutting tool be made to occupy strictly determined and controlled positions along its feed advance path in response to angular positions of the rotating tool. Also, full flexibility is needed in selecting phase .phi. and ratio 2A/p to obtain the best surface finish and the longest tool life. Additionally, it would be desirable to apply the method to all machining operations that can be performed using a rotating tool that has `n` cutting edges.